Why doesn't the water in fire hydrants freeze during winter?

Why doesn't the water in fire hydrants freeze during the winter? It would be quite a problem if firemen showed up to put out a blaze, and no water came out!

— Chris

Because there isn't any water in them to begin with.

Fire hydrants are equipped with an anti-siphon valve (meaning they are self-draining). The valve seal--the "door," so to speak--is at the bottom of the hydrant ... underground, safely below the frost line, at the point where the hydrant taps into the water main. A long rod runs from this valve to the top of the hydrant. The firefighters put a wrench on the top of the rod to open the hydrant. Once turned off, the water drains from the hydrant back into the ground around it so the barrel from the water main into the hydrant is empty. Hydrants are considered frostproof up to -50 degrees.

But the first source of water for firefighters is not the fire hydrant. It's the fire engine (as opposed to the fire truck, which carries the ladders). A standard fire engine usually holds from 500-700 gallons of water, more than enough to put out the average house fire.

Water's not the only solution to fire. At home, small fires (like the kind you get on the stovetop when you're cooking) are easily put out by removing the pan from the heat and sprinkling it with baking soda (the soda eliminates the oxygen and puts out the fire, making it a popular ingredient in dry-powder fire extinguishers).

My father, Casey, who was a firefighter in a Class One city for 30 years, says the best way to put out a fire is with common sense . Fire is made up of three things: ignition (heat), fuel and oxygen.

Take away any of these three, and the fire's out.

— Songbird

Staff Reports are written by the Straight Dope Science Advisory Board, Cecil's online auxiliary. Though the SDSAB does its best, these columns are edited by Ed Zotti, not Cecil, so accuracywise you'd better keep your fingers crossed.